Signals may be used to transmit data over distances. In optical communication systems, for example, data may be modulated on one or more optical wavelengths to produce modulated optical signals that may be transmitted over optical waveguides such as optical fibers. Optical communications systems have used techniques such as wavelength division multiplexing (WDM) and various multi-level modulation formats to increase the transmission capacity. In a multi-level modulation format, multiple data bits may be encoded on a single transmitted symbol. Multi-level modulation techniques have been used, for example, to allow increased transmission rates and decreased channel spacing, thereby increasing the spectral efficiency (SE) of each channel in a WDM system.
One type of modulation that may be used in optical communication systems is phase shift keying (PSK). According to different variations of PSK, data is transmitted by modulating the phase of an optical wavelength such that the phase or phase transition of the optical wavelength represents symbols encoding one or more bits. In a binary phase-shift keying (BPSK) modulation scheme, for example, two phases may be used to represent 1 bit per symbol. In a quadrature phase-shift keying (QPSK) modulation scheme, four phases may be used to encode 2 bits per symbol. Other phase shift keying formats include differential phase shift keying (DPSK) formats and variations of PSK and DPSK formats, such as return-to-zero DPSK (RZ-DPSK) and phase division multiplexed QPSK (PDM-QPSK).
Another spectrally efficient multi-level modulation format is quadrature amplitude modulation (QAM). According to QAM, information may be modulated using a combination of phase shift keying and amplitude shift keying, for example, to encode multiple bits per symbol. A 16-QAM modulation format may be used, for example, to encode 4 bits per symbol. Certain PSK modulation schemes (e.g., BPSK and QPSK) may be referred to as a level of QAM (e.g., 2QAM and 4QAM respectively). Although 1024QAM has been demonstrated, it is difficult to generate nQAM for n>4.
Several different schemes may be used to generate higher levels of QAM. In one scheme, tandem AM (for amplitude) and PM (for phase) modulators may be used with a multi-level RF driving signal. In another scheme, a single IQ modulator and multi-level RF driving signal may be used. In a further scheme, multiple parallel Mach-Zehnder (MZ) modulators, each driven by a binary RF signal, may be used in a nested MZ structure. In yet another scheme, tandem AM/PM/QPSK modulators may be used with binary driving signals for each modulator.
With multilevel RF driving signals, achieving uniform QAM signal constellations may be difficult due to the nonlinear transfer functions of the MZ modulators. Therefore, a binary RF driving signal may be preferred. Although a binary RF driving signal may be used with tandem AM/PM/QPSK modulators, maintaining synchronization among all modulators in such a system may be difficult. The integration of multiple parallel MZ modulators may also be difficult to achieve.